1) Field of the Invention
The present invention relates to a method of producing straight-chain acrylonitrile dimers. More particularly, the present invention relates to a method of producing straight-chain acrylonitrile dimers including 1,4-dicyanobutene, 1,4-dicyanobutadiene and adiponitrile at a high selectivity and yield.
The above-mentioned straight-chain acrylonitrile dimers are useful as an intermediate for producing hexamethylene-diamine which is an important material for producing nylon 66, rust-inhibiting agents and vulcanization promoters for rubber materials.
2) Description of Related Art
A method of producing 1,4-dicyanobutene compounds and adiponitrile by using a catalyst comprising a ruthenium compound is well-known from A. Misono et al., Bull. Chem. Soc. Jpn., Vol. 41 No. 2, 396-401 (1968). In the method, acrylonitrile is dimerized in the presence of a ruthenium catalyst in a hydrogen gas atmosphere, Misono et al., disclose that if this method is carried out not using the hydrogen gas atmosphere, no dimerization of acrylonitrile occurs, and the dimerization of acrylonitrile can proceed only within the hydrogen gas atmosphere. This method is, however, disadvantageous in that acrylonitrile is brought into contact with the ruthenium catalyst in the hydrogen gas atmosphere, wherein dimerization of acrylonitrile proceeds and hydrogenation of acrylonitrile, which is not desired, occurs as a side reaction and a large amount of propionitrile is produced as a by-product. Therefore, to utilize the by-product consisting of propionitrile for practical use, it is necessary to return propionitrile to acrylonitrile by a dehydrogenation of propionitrile in the presence of a catalyst. However, this dehydrogenation of propionitrile exhibits a low selectivity and the dehydrogenation catalyst has poor catalytic activity. Therefore, the conversion of propionitrile to acrylonitrile is difficult and it is disadvantageous to industrialize, same.
JP-B-44-24,585, JP-B-45-4,048 and JP-B-54-12,450 disclose methods of producing straight-chain acrylonitrile dimers with an enhanced selectivity. In these methods, the selectivity of the straight-chain acrylonitrile dimers is 55 to 67%. Nevertheless, since the dimerization of acrylonitrile in these methods is carried out in the presence of hydrogen, it is unavoidable that propionitrile be produced as a by-product at a high selectivity of 33 to 45%. This by-production of propionitrile in a large amount renders the methods of the above-mentioned Japanese publication for producing straight-chain acrylonitrile dimers industrially inapplicable.
JP-B-54-12,450 discloses a method of dimerizing acrylonitrile in the presence of a catalyst and hydrogen comprising an inorganic ruthenium compound, ruthenium carboxylate, or ruthenium complex. In this method, the dimerization reaction of acrylonitrile is promoted by adding, to the reaction system, a carboxylate of a specific metal selected from lead, zinc, cadmium, tin, iron and manganese. This prior art method, is disadvantageous in that since the dimerization reaction of acrylonitrile is carried out in the presence of hydrogen, the undesirable by-production of propionitrile is unavoidable.
JP-A-51-146,420 discloses that acrylonitrile can be dimerized into dinitrile at a reaction temperature of 300.degree. C. to 600.degree. C. in the presence of a ruthenium catalyst in the absence of hydrogen. Nevertheless, this method is disadvantageous in that catalytic activity is unsatisfactorily low and thus when propionitrile is produced as a by-product in a low yield, the conversion of acrylonitrile to intended dimers is also poor, for example, several percent. Where acrylonitrile is converted at a high conversion rate, the by-production of propionitrile is increased, and the selectivity of C.sub.6 dinitrile products is reduced. Further, the resultant dinitrile product comprises a mixture of straight-chain dimers and branched-chain dimers. Therefor, the selectivity of the intended straight-chain dimers is unsatisfactorily low.
Accordingly, there is a strong demand for the provision of a method of producing straight-chain acrylonitrile dimers while restricting the undesirable production of propionitrile that cannot be easily converted to acrylonitrile.
In the prior art methods, the use of hydrogen causes an undesirable high risk of explosion when the reaction is mixed with air or oxygen, and thus a complete sealing of the reaction system is necessary.
The above-mentioned causes the reaction procedures and apparatus to be complex and costly.
Therefore, there is a strong demand for the provision of a new method of safely producing straight-chain acrylonitrile dimers without using hydrogen.